The present invention relates generally to optical detection systems and, more particularly, to an optical detection system for monitoring interference patterns generated in response to temporally coherent light sources of widely varying intensities in the presence of widely varying background light levels.
Temporally coherent light, such as from a laser, is utilized to determine the range to a target or to guide a weapon to that target. Hence, in order to counter such ranging and weapons deliver systems, it is first necessary to determine whether laser light is incident upon a potential target and, if present, the location of the source of the laser light. For detection, incident light is directed to an interferometer which generates an interference pattern upon illumination by temporally coherent light. The interference pattern is monitored by an array of light detecting elements which generate voltage or current signals representative of the light incident upon the respective elements.
Background signals due to dark current integration in the array of light detecting elements or ambient incoherent illumination, such as sunlight, can be a problem. In the optical detecting system of the present invention, background signals occur with wide variations in intensity. Such wide variations are encountered since a system for detecting laser light used for ranging or weapon guidance must scan the sky whether installed on ground-based equipment or on aircraft. Under such circumstances, the light receiver for the system may look nearly directly into the sun. The intensity of the coherent light is similarly widely varying since it may be directly incident upon the light receiver of the detecting system or may be received indirectly by atmospheric scattering.
One solution to excessive background signals is to have processing circuitry subtract an average of the output signals of all the detector elements of an array from the signal of each detector element prior to further processing. For example, in the prior art, all of the voltage signals from a plurality of individual detector elements are summed by an operational amplifier and the sum is divided by the number of elements to generate the average voltage level of the voltage signals. The average voltage level is then subtracted from each of the signals from the detector elements to determine which of the detectors are sensing light fringes (element voltage signal exceeds the average voltage level) and which of the elements are sensing dark fringes (average voltage level exceeds the element voltage signal) of the interference pattern.
In the worst case, coherent and/or incoherent light signals can become so large that they drive the averaging amplifier associated with the detector array into saturation in spite of such measures. If this occurs, then all information is lost and the detecting system is effectively disabled. The problem of excessive background signals can be recognized to be in the circuitry which processes the light detector output signals since the light detectors themselves, particularly semiconductor photodiodes, have an enormous dynamic range exceeding 100 dB for some devices.
The need therefore exists for an improved system which permits the detection and location of an interference pattern to be determined where the interference pattern is generated in response to temporally coherent light which has widely varying intensities and is intermixed with background light also having widely varying intensities.